The present invention relates to a plasma ion source mass analyzing apparatus for specifying and measuring minute impurities in a sample. The term "plasma ion source mass analyzing apparatus" includes an inductive coupling plasma mass analyzing apparatus (referred to as ICP-MS) and a microwave induction plasma mass analyzing apparatus (referred to as MIP-MS).
An example of an arrangement according to the prior art will be explained with reference to FIG. 4. In FIG. 4, reference numeral 1 denotes a plasma generation apparatus, and numeral 2 denotes a plasma. The plasma generation apparatus may be, for example, an inductive coupled plasma generation apparatus disclosed in "ICP LIGHT EMISSION ANALYZER AND ITS APPLIANCE" by Haraguchi, Kodansha Scientific, or for example, a microwave plasma generation apparatus disclosed in Japanese Patent Application Laid-Open No. Hei 1-309360 (USP 4,933,650).
A sample (not shown) to be analyzed is introduced into the plasma 2 generated by the plasma generation apparatus 1 to be ionized. Numeral 3 denotes a sampling cone, numeral 4 denotes a skimmer cone, and numeral 5 denotes a vacuum pump. The sampling cone 3 which is provided at its tip with an opening having a diameter of 0.8 to 1.2 mm. The skimmer cone 4 is provided at its tip with an opening having a diameter of 0.3 to 0.6 mm. The sampling interface is composed of the sampling cone 3 and the skimmer cone 4. A space between the sampling cone 3 and the skimmer cone 4 is evacuated down to about 1 Torr by the vacuum pump 5 (for which a rotary pump is generally used) during the analysis.
Numeral 6 denotes a vacuum container, numeral 7 denotes an ion lens, numeral 8 denotes a mass filter, numeral 9 denotes a detector, and numeral 12 denotes a data processor. The interior of the vacuum container 6 is evacuated by two different vacuum pumps 5 and 5 and is maintained in a vacuum condition of about 10.sup.-4 Torr in a chamber where the ion lens 7 is disposed and in a vacuum condition of about 10.sup.-6 Torr in a chamber where the detector 9 is disposed. In general, turbo molecular pumps or oil diffusion pumps are used as these vacuum pumps 5 and 5.
The sample which has been ionized by the plasma 2 reaches the ion lens 7 through the openings of the sampling cone 3 and the skimmer cone 4 together with light of the plasma. The ion lens 7 serves to introduce, into the mass filter 8, only the ions, out of all the ions and light, which have reached the ion lens. The mass filter 8 serves to pass only a predetermined mass of ions out of all the ions which have reached the mass filter 8. For example, a quaternary pole mass analyzer is used as the mass filter 8.
The detector 9 detects the ions which have passed through the mass filter 8 and sends a corresponding electric signal to the data processor 12. For example the detector 9 may be a commercially available device, such as that sold under the trademark CHANNELTRON produced, a Channeltron made by Galileo company. In the data processor 12, the mass of the ions is calculated from setup values of the mass filter 8 when it is detected by the detector 9 and the type of ion is determined. Then, the data processor 12 calculates the concentration of the ions specified by the detecting strength of the detector 9, i.e., the impurities contained in the sample.
The ion lens 7 will now be explained with reference to FIG. 5. FIG. 5 is a schematic cross-sectional view of the ion lens and its vicinity. Numeral 13 denotes a sampling interface axis, characters 14a, 14b and 14c denote electrodes, characters 15a and 15b denote deflectors, numeral 16 denotes an aperture, and numeral 17 denotes a mass filter axis. The ion lens 7 is composed of the electrodes 14a, 14b and 14c, the deflectors 15a and 15b and the aperture 16.
The sampling interface axis 13 extends through centers of the opening of the sampling cone 3 and the opening of the skimmer cone 4. An ion beam which has passed through the opening of the skimmer cone 4 reaches the ion lens 7 along the sampling interface axis 13. A convergent lens is formed by the three electrodes 14a, 14b and 14c each of which is in the form of a plate having an opening at its center along the sampling interface axis 13. When suitable voltages are applied to the electrodes 14a, 14b and 14c, respectively, the beam is converged. Such a convergent lens is referred to as an Einzel lens.
The master filter axis 17 corresponds to an optical axis which is reached by the ion beam converged to the mass filter 8. The mass filter axis 17 is located in parallel with an interval of about 10 mm relative to the sampling interface axis 13. The aperture 16 is in the form of a plate having an opening about the mass filter axis 17. When a suitable voltage is applied thereto, the aperture serves to send the ion beam having a suitable energy to the mass filter 8. The aperture 16 is not necessarily a single opening and may be instead composed of a plurality of elements. For example, the deflectors 15a and 15b are composed of planar parallel type deflectors, respectively. The deflectors 15a and 15b cause the ion beam, which has been converged along the sampling interface 13, to pass through the mass filter axis 17. Namely, they serve to deflect the converged ion beam.
The ion lens 7 thus arranged serves to introduce into the mass filter 8 the ion beam to be detected as described above, and at the same time serves to prevent the light of the plasma 2, which adversely affects the detector 9 as a background noise, from reaching the mass filter 8 by causing the light to advance in the ion lens 7 and collide against the aperture 16.
Since a neutral component which has not completely been ionized by the plasma 2 is present in addition to the abovedescribed ions and the light produced by the plasma 2 is also present as a component which pass through the skimmer cone 4, the following problems are noticeable. The neutral component is forwardly advanced as is the light in the ion lens 7 to collide against the aperture 16 and from a film. The main component of the neutral component is a structural component of the sample, and the film stuck to the aperture 16 hardly has electric conductivity. The film is then, however, charged to have an unstable surface potential. Namely, if this film is stuck to the ion lens 7, an electric field in the interior of the ion lens 7 becomes unstable, a path of the beam of ion to be detected is unstable and as a result, it is impossible to effect a stable measurement. In the prior art, due to an excessive influence exerted by the film troublesome and time consuming work has to be carried out requiring that the ion lens be removed while stopping the apparatus to permit it to be dismantled and cleaned.